1. Technical Field
This invention relates to an apparatus and a method for the manufacture of engineered wood products, and more particularly to the use of microwaves to accelerate the curing of resins used in engineered wood products.
2. Background
Engineered wood products are made by combining wood fibers and a resin which hardens as it cures and binds the fibers together.
Traditionally, wood fiber in the form of layers of veneer or pieces of wood fiber of various sizes, have been made by being pressed together in a heated press. The heat from the press is transmitted to the wood fibers and binding material in the press by simple heat conduction from the press platens into the wood. As the binding material is heated, its curing time is decreased. After a certain amount of time at a certain temperature and pressure, the binding material is fully cured and may be released from the press. Before the binding agent has fully cured, the wood fibers and binding agent are placed under pressure in a press in order to put as much wood fiber in contact with the binding agent as possible. When pressed in this way and then hardened, the resulting product has the maximum strength and durability properties obtainable.
Since wood is a good insulator, transferring heat through wood by conductance has certain limitations. As the thickness of a piece of wood being heated and pressed increases, the amount of time that it takes in the press to transmit heat to the center of the work piece also increases. Beginning in the 1930's, it was found that radio frequency (RF) energy could be used successfully to pass energy through layers of wood and glue in order to heat the interior mass and cause the glue to cure faster. Some ways of applying RF and microwave energies to these products were in devices which are similar to a giant waffle iron through which RF energy is passed from one plate to another through the engineered lumber "waffle". Another method is to form a billet of material consisting of wood veneer strands combined with adhesive, then placing the billet in a press and squeezing it from the top, bottom and two sides, and while under pressure, illuminating the interior of the billet with microwaves which are directed from one or both sides of the billet. In order to resist the pressure applied by the press, microwave energy which is applied through the sides of the billet enters the press chamber through a window which is strong enough to withstand the pressures of the press, and which is also transparent to microwave energies.
Microwaves heat the billet during such a pressing operation by excitation and rotational oscillatory movement of polar molecules, such as water molecules, inside the billet caused by the oscillating electric fields that are part of the microwave signal.
As the microwave signals strike a wood product prior to and during pressing, a portion of the microwaves are reflected back toward the microwave source which originally produced the microwaves. This reflective signal is usually channeled to a dissipating dummy load that is connected to a device in the microwave source itself. This reflected and dissipated microwave power is wasted and is not used in the heating of the wood product. RF energy is similarly directed into a billet of engineered wood material. RF energy is carried directly into the lay-up assembly or billet where it excites the polar molecules in the materials of the lay-up assembly. This interaction generates heat in the polar molecules which causes the shortening of curing times for binding agents.
However, a problem that has been encountered with the use of RF energy is that when RF is directed into a billet of veneer and glue layers in a direction parallel to the glue lines, and where the glue used is an alkaline solution of phenol formaldehyde resin, which is the most common of binding agents, the energy can cause arcing and tracking, especially along the layer of glue. The thicker the layer of glue, or the higher the water content of the glue, the more that the arcing and tracking becomes a problem. The reason for this undesirable effect is a relatively high conductivity of the resin which can lead to breakdown as the electric field from the microwave is integrated along a single axis. The arcing problem is greatly reduced if the electric field is applied perpendicular to the planes formed by the wood veneer layers and the layers of glue between them.
Another problem encountered in making engineered wood products is that energy directed into the billet while it is under pressure can cause moisture within the layers of wood to flash or boil away rapidly. When the pressure on the billet is released, if the pressure from expanding gasses is greater than the strength of the binding material holding the wood fibers together, the expanding gasses can cause a blowout.
Still another problem encountered in making engineered wood products which are heated by microwaves directed from the side of the billet toward the center of the billet while the billet is under pressure in a press is that the width of material through which the microwave energy can pass so that the center of the material is heated is limited. Billets which are very much wider than 24 inches are difficult to heat from side applied microwave energy. If these billets are not only wide in the lateral dimension, but also thick in the dimension normal to the longitudinal axis, they are also difficult to heat by conduction from the press platens because of their thickness. Therefore, the thickness of billets is limited by the prior art techniques of heating through conduction from the press platens and side directed microwave energy in the press.
Another problem with the current technology of preparing engineered wood products is that the process is fairly sensitive to variations in moisture content. Since the wood itself can have wide variations in density and moisture content, a common practice is to dry the wood to a uniform and low moisture content, and then to add back enough water to bring the wood fibers to the preferred moisture content. This preparation of the wood fiber is expensive and time consuming.
Accordingly, it is an object of the invention to provide a means by which wide work pieces can be uniformly heated by microwave energy, and in which width is not a factor or limitation. Another object of the invention is to provide a microwave heating system in which water vapor from the work piece can escape, decreasing the possibility for blow outs in the wood fiber.
A further object of the invention is to provide a system which can accommodate a greater variation in the moisture content of the wood fibers than permitted in the prior art. Related to the ability to operate with more variation in the moisture content of the wood fiber, it is an object of the current invention to operate at a reduced price due to reduced expenses of preparation of the wood fiber materials.
It is a further object of the invention to provide a microwave heating system which provides for maximum efficiency in the use of microwave energy.
It is a further object of this invention to be able to heat a billet of fibrous material to a given temperature, such that the heat is evenly distributed throughout the billet, or can be maximized in the center of the billet or another region of the billet as chosen by the operator. As a result of this capability, a further object of the invention is to increase the volume which can be processed through an engineered wood press due to the press time being decreased by the use of the microwave heating system of the invention.